Processing of metallic charge-transfer salts

ABSTRACT

Metallic salts of organic charge-transfer agents, such as TCNQ, TNAP, TCNE and DDQ and their derivatives, can be processed by an electron beam for a variety of useful electronic and optical applications. The metallic charge transfer salts can be used to deposit high resolution conductive lines directly without developing solutions or subsequent metallization steps. The compounds can also be employed in the conventional manner as resists for doping (i.e., ion diffusion or implantation) and to diffuse metals into substrates. In particular, electronic devices, optical devices and image-storage devices are disclosed which can be formed by simple electron beam processed of metal charge-transfer salt films deposited on substrates.

BACKGROUND OF THE INVENTION

This invention relates to the processing of metallic salts of chargetransfer agents as well as devices formed by such methods. Inparticular, techniques for processing metal-tetracyanoquinodimethane(TCNQ) salts, metal-tetracyanonaphthoquinodimethane (TNAP) salts,metal-tetracyanoethylene (TCNE) salts, dichlorodicyanobenzoquine (DDQ)salts and similar metallorganic semiconductor salts are disclosed toform resists, active electronic devices and image-storage devices, andto dope substrates and modify the bulk or surface properties ofmaterials.

Organic electron-acceptors and their metallic salts have been studied bynumerous researchers. Cuprous and other metallic salts of TCNQ, forexample, were disclosed by Melby et al. in "Substituted QuinodimethanesII. Anionradical Derivatives and Complexes of 7,7,8,8Tetracyanoquinodimethane," Vol. 84, J. of American Chemistry, pp.3374-3387 (1962). The electrical switching properties of Cu-TCNQ filmswere disclosed by Potember et al. in article "Electrical Switching andMemory Phenomena in Cu-TCNQ Thin Films," Vol. 34, Applied PhysicsLetters, pp. 405-407 (1979) and further described by the presentinventors and a colleague in an article "Raman Study of the Mechanism ofElectrical Switching in Cu-TCNQ Films," Vol. 42, Solid StateCommunications, pp. 561-565 (1982). The optical properties of such filmswere disclosed by the present inventors in an article "Optically InducedTransformations of Metal TCNQ Materials," Vol. 45, Solid StateCommunications, pp. 165-169 (1983). The teachings of the abovereferenced articles are incorporated herein by reference.

Despite the teachings in this art, few commercial applications ofmetallic-TCNQ salts and the like have been found to date. In the fieldof semiconductor processing, for example, there currently is asignificant need for resist materials that can be patterned anddeveloped to yield sub-micron structures for active electronic devices.Similarly, there is a need for better memory materials for bothreversible and "read-only" information storage devices. High resolutionimage storage is yet another area where imaging materials of highquality would satisfy a substantial need.

SUMMARY OF THE INVENTION

It has been discovered that metallic salts of organic charge-transferagents, such as TCNQ, TNAP, TCNE and DDQ and other electron acceptors,can be processed by electron beams for a variety of useful electronicand optical applications. In one aspect of the invention, metal chargetransfer salts are employed to deposit high resolution conductive linesdirectly without developing solutions or subsequent metallization steps.The compounds disclosed herein can also be employed in conventionalmanners as resists to form masks or patterns upon semiconductorsubstrates for doping (i.e., ion diffusion or implantation) or otherelectronic device fabrication steps.

Electron beam processing sufficient to cause the dissociation of themetal-organic salt into its free valent species can also be used todeposit a controlled amount of a dopant metal in the processed region.When the system is heated, the metal atoms so placed form a source ofdoping atoms which can be diffused into the substrate as dopants. Bythis method, controlled doping of semiconductors, metals and insulatorscan be achieved in the pattern of exposure to the electron beam. Thepreferred embodiment depends on the desired modification and theproperties of the substrate and the chemical properties of thesubstrate, salt and etchant. For example, in the case of LiTCNQ as thefilm, lithium doping can be achieved by thermal diffusion in theprocessed regions and the unprocessed LiTCNQ can be removed usingsolvents such as acetonitrile. As another example, in the case of AgTCNQas the thin film, Ag doping to modify the bulk or surface properties ofthe substrate can be achieved by thermal diffusion in the processedregions and the unprocessed AgTCNQ is removed by other etchants.

In another aspect of the invention, the processing techniques are usedto form electronic devices. Electrically controlled bistable thresholdand memory switches can be fabricated by electron beam treatment offilms formed from metallic charge transfer salts to yield in situelectrodes in those regions of the film exposed to electron bombardment.Structures formed in this manner include, for example, semiconductormaterials (i.e., the unexposed metallic charge transfer salt) sandwichedbetween two electrodes. The semiconductor can be switched from a highimpedance state and a low impedance state by applying an electric field.The threshold for switching or memory behavior is dependent upon theparticular transfer agent and metal chosen, the thickness of the filmand the temperature. The duration of the switching or memory behavior isalso dependent on the material, thickness and temperature as well as thestrength of the applied field. One important advantage of devicesfabricated by the electron processing techniques of the presentinvention is that the devices so formed are essentially planar and donot require multiple masking steps for electrode deposition ormetallization of contacts.

In a further aspect of the invention, electron beam processing can alsobe employed to form light activated switches and memory elements. Uponexposure to light, planar sandwich structures with in situ formedelectrodes, can also be switched from a high impedance state to a lowimpedance state. Such optical devices can find use in sensors, memoryelements, optical computers and optical communications systems. In theseapplications the threshold and duration of the switching or memorybehavior will be depend not only on materials, thickness and temperaturebut also on the wavelength and energy of the light.

In yet another aspect of the invention, it has been discovered thatmetallic charge transfer salts deposited on suitable substrates can beprocessed by electron beams to generate very high resolution hologramsand the like. Selective activation of the electron beam to dissociatethe salt and volatize the organic component in a pixel-by-pixel manneris employed to produce holographic images. Processing of pixels on theorder of one micron or less is achievable. In one embodiment themetallic charge-transfer salt is deposited as a film on a highlyreflective substrate such as a silver or copper mirror surface. In thoseareas where the electron beam is activated, the constituents of the saltare converted to free valent species, and the organic component isallowed to escape by sublimation. The resulting pattern of salt andmetal yield a reproduced image by reflection upon illumination. Inanother embodiment, a glass, quartz or other transparent substrate canbe employed to form holographic images suitable for viewing bytransmittance or projection (i.e., like photographic slides). Hologramformation by the techniques disclosed herein should not only allowbinary (black or white) processing of each pixel but also permitgradation in shading by appropriate controls over the duration ofelectron beam exposure.

The mechanisms underlying the present invention consist of two mainreactions. The first mechanism involves the reversible dissociation ofthe salt constituents into their free valent species which is depictedfor partially transformed one-to-one metal charge transfer salts as:

    (M.sup.+ CT.sup.-).sub.n ⃡(M.sup.o).sub.x +(CT.sup.o).sub.x +(M.sup.+ CT.sup.-).sub.n-x

where M is the metal constituent and CT is the organic charge transferagent. This reaction is known to occur in the presence of an appliedelectric field or exposure to light of appropriate wavelength andenergy. This reaction is used herein to achieve the threshold and memoryswitching behavior of the electronic and optical devices describedbelow. Non-planar electronic switches employing the same mechanisms havebeen disclosed, for example, in U.S. Pat. No. 4,371,883 issued toPotember et al. on Feb. 1, 1983, the teachings of which are incorporatedherein by reference.

The second mechanism, which forms the basis for the electron beamprocessing steps disclosed herein, involves the irreversible dissocationof the salt constituents. In this reaction the constituents are not onlyconverted into free valent species but the organic constituent sublimesto leave a metal deposit in those areas exposed to electron bombardment:

M⁺ CT⁻ →M^(o) (at film)+CT^(o) (lost to vapor)

The charge transfer agents useful in the present invention includeorganic electron acceptors, such as tetracyanoquinodimethane (TCNQ),tetracyanonaphthoquinodimethane (TNAP), tetracyanoethylene (TCNE),dichlorodicyanobenzoquinone (DDQ), and derivatives of these compounds.Metals useful in the present invention include copper, silver, platinum,palladium, nickel, cobalt, zinc, cadmium, iridium, osmium, and rhodium.Additionally, lithium, potassium and similar metals can also be usefulas deposited dopant sources. More generally, metals from Groups 1, 2,11, 12 and 13 of the periodic table of elements (new IUPAC-ACSclassification) can form charge transfer salts useful in practice of thepresent invention. Specifically, the copper and silver salts of chargetransfer agents identified in Table 1 below are illustrative of variousderivatives and can find use in particular applications.

                  TABLE 1                                                         ______________________________________                                        CuTCNQ            AgTCNQ                                                      CuTCNQ(OMe)       AgTCNQ(OMe)                                                 CuTCNQ(OMe).sub.2 AgTCNQ(OMe).sub.2                                           CuTCNQ(OMe)(OEt)  AgTCNQ(OMe)(OEt)                                            CuTCNQ(OMe)(O--i-Pr)                                                                            AgTCNQ(OMe)(O--i-Pr)                                        CuTCNQ(OMe)(O--i-Bu)                                                                            AgTCNQ(OMe)(O--i-Bu)                                        CuTCNQ(O--i-C.sub. 2 H.sub.5)                                                                   AgTCNQ(O--i-C.sub. 2 H.sub.5)                               CuTCNQ(OEt)(SMe)  AgTCNQ(OEt)(SMe)                                            CuTCNQClMe        AgTCNQClMe                                                  CuTCNQBrMe        AgTCNQBrMe                                                  CuTCNQlMe         AgTCNQlMe                                                   CuTCNQCl          AgTCNQCl                                                    CuTCNQBr          AgTCNQBr                                                    CuTCNQI           AgTCNQI                                                     CuTCNQ(Ome)(OCH.sub.3).sub.2                                                                    AgTCNQ(Ome)(OCH.sub.3).sub.2                                CuTCNQ(CN.sub.2)  AgTCNQ(CN).sub.2                                            CuTCNQ(Me)        AgTCNQ(Me)                                                  CuTCNQ(Et)        AgTCNQ(Et)                                                  CuTCNQ(i-Pr)      AgTCNQ(i-Pr)                                                CuTCNQ(i-Pr).sub. 2                                                                             AgTCNQ(i-Pr).sub. 2                                         CuTCNQ--F.sub.2   AgTCNQ--F.sub.2                                             CuTCNQ--F.sub.4   AgTCNQ--F.sub.4                                             CuTNAP            AgTNAP                                                      CuTCNE            AgTCNE                                                      CuDDQ             AgDDQ                                                       ______________________________________                                    

The invention will next be described in connection with certainpreferred embodiments; however, it should be clear various changes andmodifications can be made without departing from the spirit or scope ofthe invention. For example, in the illustrations described below, themetallic charge-transfer salts are preferably formed by first depositingalternating layers of metal and the charge transfer agent, and thenconverting the two constituents into a salt by heating. It should beclear that other methods of salt formation can be substituted, such aschemical transformations and depositions from solutions. Additionally,the electronic and optical structures described herein can beincorporated into larger systems and devices. For example, memory cellsemploying the devices described below can include conventional circuitryfor reading the memory state as well as setting and resetting thedevice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1c are schematic, cross-sectional illustrations of varioussteps in the processing of metallic charge-transfer salts according tothe invention.

FIGS. 2a-2c are schematic, cross-sectional illustrations of furthersteps in the processing of metallic charge-transfer salts according tothe invention.

FIGS. 3a-3c are also schematic, cross-sectional illustrations of furthersteps in the processing of metallic charge-transfer salts according tothe invention.

FIG. 4 is a schematic, cross-sectional illustration of an electronicswitch or memory device fabricated according to the invention.

FIG. 5 is a schematic, cross-sectional illustration of an optical switchor memory device fabricated according to the invention.

FIG. 6 is a schematic, prospective view of a holographic engraving andviewing system according to the invention; FIG. 6a is a more detailedschematic view of a holographic plate prepared according to theinvention.

DETAILED DESCRIPTION

In FIGS. 1a-1c the basic steps in electron beam processing according tothe invention are shown. In FIG. 1a, one preferred technique forformation of the metallic charge-transfer salt by vapor deposition asshown. Alternating layers of the charge-transfer agent 10 and metal 12are deposited upon a substrate 14. For copper and silver salts, such asCuTCNQ and AgTCNQ, it is typically preferred that the molar ratio of thetwo constituents be maintained close to 1:1 but ratios in the range of1:1 to 1:3 (metal:charge transfer agent) can be appropriate for othermetal salts and excesses of either metal or charge transfer agent can beincorporated for special purposes. Because of the density differencesbetween the organic and metal layers, the organic layers aresubstantially thicker. For example, in the case of the formation ofCuTCNQ, alternating layers of about 20-200 angstroms Cu and 400-5000angstroms TCNQ are deposited on the substrate. Although the schematicdrawing shows only three layers, it should be appreciated that thenumber of layers deposited in this manner can be quite large. It is alsopreferred that the initial and final deposition layers be metal layers12 to increase the integrity of the structure shown in FIG. 1a duringsubsequent processing steps. The substrate can be a conductor,semiconductor or insulator, including materials such as quartz, glass,copper, silver, potassium bromide, silicon or germanium.

In FIG. 1b, the conversion of the layered structure to a salt is shownschematically. Such conversion occurs readily when the layered structureis heated at a particular temperature and for a particular time. Forexample, in the case of CuTCNQ heating at about 90° C. or above for afew minutes is sufficient for complete conversion of the salt. If excessTCNQ is present, it can be removed by solvent wash at this time. Theheat-induced conversion will occur in a vapor deposition chamber orunder an inert atmosphere. In the case of some of the metals, such ascopper or silver, it is possible to conduct the conversion in an openair oven if the layered structure is not exposed to air for more than afew minutes handling time. In the case of more reactive or less easilypassified metals, exclusion of air, water or other reactive species isrecommended and the preferred method is reaction in the vapor depositionchamber followed by washing out any excess charge transfer agent.

In FIG. 1c the effects of electron beam processing are shownschematically. In those regions of the salt 20 which are exposed toelectron beam 22, the salt decomposes into a constituent free valentspecies. The energy of the electron beam is sufficient to cause thesublimation of the organic charge transfer agent, leaving only the metalconstituent 24. Vacuum conditions are recommended to encouragesublimation so that an electron beam source of lower power and intensitycan be used. Metal regions on the substrate surface, formed in thismanner can be utilized in semiconductor fabrication for conductive linesand contact points.

In FIGS. 2a-2c further steps in processing metallic charge-transfersalts according to the invention are shown. The structure shown in FIG.2a is essentially the same as that shown in FIG. 1c, that being theresult of electron beam processing. The metal regions can be removed asshown in FIGS. 2b and 2c by first oxidizing the metal to yield a metaloxide or other salt 26 and then etching this oxide or salt away with asuitable etchant, such as an acid or basic etch solution. Because themetal oxide or salt is substantially more succeptable to particularetchants, the metal regions are selectively removed leaving thesalt-covered regions of the substrate intact. The substrate can then bedoped by conventional diffusion or implantation techniques in theexposed regions or other standard fabricating techniques such as thegrowth of different layers, the etching of the substrate, itself, orlift-off processes can be carried out. The remaining salt resist on thesubstrate can be removed by more powerful etching solutions or byfollowing the steps described in FIGS. 2a-2c again after the furtherfabrication steps have been conducted on the exposed regions of thesubstrate.

FIGS. 3a-3c also illustrate further steps in processing metalliccharge-transfer salts according to the invention. The structure shown inFIG. 3a is essentially the same as that shown in FIG. 1c, that being theresult of electron beam processing. By heating the substrate as shown inFIG. 3b, the deposited lithium, potassium or similar metal regions canbe thermally diffused into the substrate as a dopant or the like. Forexample, in the case of lithium doping, heating the substrate to about150° C. or above can induce diffusion. After diffusion the remainingmetal and salt can be removed by suitable etchants (e.g., acetonitrilefor LiTCNQ) as shown in FIG. 3c. As another example, in the case ofAgTCNQ as the thin film, Ag doping to modify the bulk or surfaceproperties of the substrate can be achieved by thermal diffusion in theprocessed regions and the unprocessed AgTCNQ is removed by otheretchants.

In FIG. 4 an illustrative embodiment of an electronic device 30fabricated according to the present invention is shown comprising asubstrate 14, a semiconductor material 32, formed from an unexposedmetallic charge-transfer salt, a first electrode 34 and a secondelectrode 36. The structure can be formed from a single film of metalliccharge-transfer salt in which the first and second electrodes,sandwiching the semiconductor, are fabricated by exposure to an electronbeam. Contacts 34a and 36a are shown schematically, applying current toelectrodes 34 and 36, respectively. The device shown in FIG. 4 can be atwo terminal threshold or memory switch which is stable in either a highor low impedance state. The transition from the high to low impedancestate occurs when the electric field which exceeds a threshold level isapplied across the semiconductor 32. The field can be easily generatedby providing a voltage, by any of various known means, across the twoelectrodes 34 and 36. As a memory switch, the device 30 remains in thelow impedance state after the initial applied field (which exceeds thethreshold) is removed. As a threshold switch, the device 30 immediatelyreturns to a high impedance state when the applied field falls below aminimum holding value.

In FIG. 5, an optical threshold or memory switch 40 is shown which alsocan be a two-terminal device which is stable in either a high or lowimpedance state. The optical device 40 comprises a semiconductormaterial 42 (again formed from the unexposed salt), sandwiched between afirst electrode 44 and a second electrode 46 (both formed from theelectron-exposed regions of the salt). Contacts 44a and 46a are shownschematically providing current to electrodes 44 and 46, respectively.An encapsulation layer 48 is recommended to prevent the loss of thecharge-transfer agent during repeated duty cycling. The encapsulationlayer 48 can be formed, for example, as an evaporated layer of atransparent insulator, semiconductor or conductor depending upon theapplication. Preferred encapsulating materials include oxides such assilica (e.g. by quartz evaporation), alumina, tin oxide and indium tinoxide. As an optical threshold or memory switch, the device 40 shown inFIG. 5 can operate in either a high or low, impedance state dependingupon exposure to light. One preferred wavelength range for light-inducedimpedance transitions is visible light from about 300 to about 800nanometers, which includes important absorption bands of CuTCNQ, AgTCNQand their analogs. As a memory switch, the device 40 remains in the lowimpedance state after the light is removed. As a threshold switch, thedevice 40 immediately returns to a high impedance state when the lightis removed.

In FIG. 6 the use of the present invention to generate high resolutionholograms is shown schematically. As shown in more detail in FIG. 6a, aholographic plate 50 comprised of a substrate 14 and a salt film 20 isselectively exposed pixel-by-pixel via electron beam source 52. In thoseareas where the electron beam is activated, the constituents of the saltare converted to free valent species, and the organic component isallowed to escape by sublimation. The holographic image can be read byillumination via light source 54 to obtain a reflected image. When thereflection-mode is employed, it is preferred that the substrate 14 be ahighly reflective material such as a silver or copper mirror surface.Alternatively, substrate 14 can be glass, quartz or another transparentmaterial and illumination source 54a can be employed to obtain aholographic image for viewing by transmittance or projection.

We claim:
 1. A method of processing a metallic charge transfer salt, the method comprising:(a) applying a film of the metallic charge transfer salt onto a substrate; and (b) exposing the film to an electron beam in a predetermined pattern, the beam having sufficient energy and resolution to convert the salt in exposed regions into the free valent species of its constituents and cause the sublimation of the organic charge transfer constituent.
 2. The method of claim 1 wherein the salt comprises a metal constituent chosen from the group of copper, silver, lithium, potassium, nickel, cobalt, zinc, cadmium, platinum, palladium, iridium, osmium and rhodium and an organic electron acceptor constituent chosen from the group of tetracyanoquinodimethanes, tetracyanonapthoquinodimethanes, tetracyanoethylenes, dichlorodicyanobenizoquinones and derivatives thereof.
 3. The method of claim 2 wherein the salt is CuTCNQ.
 4. The method of claim 2 wherein the salt is AgTCNQ.
 5. The method of claim 1 wherein the method further includes removing the metal from the exposed regions of the substrate by oxidizing the metal and treating the substrate with a solvent that selectively dissolves the oxidized metal.
 6. The method of claim 1 wherein the method further includes diffusing the metal in the exposed regions into the substrate.
 7. A method of forming an electronic device, the method comprising:(a) applying a film of a metallic charge transfer salt having a organic charge-transfer constituent onto a substrate; (b) exposing a first region of the film to an electron beam to convert the first region to a first metal electrode by dissociating the salt and causing the sublimation of the organic charge transfer constituent; and (c) exposing a second region of the film to an electron beam to convert the second region to a second metal electrode by dissociating the salt and causing the sublimation of the organic charge-transfer constituent, the first and second regions being chosen such that the first and second electrodes define a sandwich structure having a region of the unexposed film disposed therebetween.
 8. The method of claim 7 wherein the salt comprises a metal constituent chosen from the group of copper, silver, lithium, potassium, nickel, cobalt, zinc, cadmium, platinum, palladium, iridium, osmium and rhodium and an organic electron acceptor constituent chosen from the group of tetracyanoquinodimethanes, tetracyanonapthoquinodimethanes, tetracyanoethylenes, dichlorodicyanobenzoquinones and derivatives thereof.
 9. The method of claim 8 wherein the salt is CuTCNQ.
 10. The method of claim 8 wherein the salt is AgTCNQ.
 11. A method of forming a holographic image of an object, the method comprising:(a) applying a film of a metallic charge transfer salt onto a substrate; (b) exposing regions of the film to an electron beam in a pattern determined by the image, the beam having sufficient energy and resolution to convert the salt in exposed regions into the free valent species of its constituents and cause the sublimation of the organic charge transfer constituent.
 12. The method of claim 11 wherein the salt comprises a metal constituent chosen from the group of copper, silver, nickel, cobalt, zinc, cadmium, platinum, palladium, iridium, osmium, and rhodium and an organic electron acceptor constituent chosen from the group of tetracyanoquinodimethanes, tetracyanonapthoquinodimethanes, tetracyanoethylenes, dichlorodicyanobenzoquinones and derivatives thereof.
 13. The method of claim 12 wherein the salt is CuTCNQ.
 14. The method of claim 12 wherein the salt is AgTCNQ.
 15. The method of claim 11 wherein the substrate is a reflective substrate.
 16. The method of claim 15 wherein the substrate includes a copper mirror surface.
 17. The method of claim 15 wherein the substrate includes a silver mirror surface.
 18. The method of claim 15 wherein the substrate is a transparent substrate. 